In the old descriptions of the disease it was frequently mentioned that large numbers of dead rats were found when it was prevalent, and the most striking fact of the recent investigations is the demonstration that the infection in man is due to transference of the bacillus from infected rats. There are endemic foci of the disease where it exists in animals, the present epidemic having started from such a focus in Northern China, in which region the Tarabagan, a small fur-bearing animal of the squirrel species, was infected. Rats are easily infected, the close social habits of the animal, the vermin which they harbor, and the habits of devouring their dead fellows favor the extension of infection. The disease extends from the rat to man chiefly by means of the fleas which contain the bacilli, and in cases of pneumonic plague from man to man by means of sputum infection. The disease once established in animals tends to remain, the virus being kept alive by transmission from animal to animal, and the persistence of the infection is favored by mild and chronic cases.
Chapter IX
Disease Carriers.—The Relation Between Sporadic Cases Of Infectious Disease And Epidemics.—Smallpox.—Cerebro-Spinal Meningitis.—Poliomyelitis.—Variation In The Susceptibility Of Individuals.—Conditions Which May Influence Susceptibility.—Racial Susceptibility.—Influence Of Age And Sex.—Occupation And Environment.—The Age Period Of Infectious Diseases.
We have seen that insects serve as carriers of disease in two ways: in one, by becoming contaminated with organisms they serve as passive carriers, and in the other they undergo infection and form a link in the disease. The more recent investigations of modes of transmission of infectious diseases have shown that man, in addition to serving while sick as a source of infection, may serve as a passive carrier in two ways. For infection to take place not only must the pathogenic organism be present, but it must be able to overcome the passive and active defences of the body and produce injury. Pathogenic organisms may find conditions favorable for growth on the surfaces of the body, and may live there, but be unable to produce infection, and the individual who simply harbors the organisms can transmit them to others. Such an individual may be a greater source of infection than one with the disease, because there is no suspicion of danger. The organisms which thus grow on the surfaces have in some cases been shown to be of diminished virulence, but in others have full pathogenic power. Such passive carriers of infection have been found for a number of diseases, as cerebro-spinal meningitis, diphtheria, poliomyelitis and cholera. In all these cases the organisms are most frequently found in those individuals who have been exposed to infection as members of a family in which there have been cases of disease. The other sort of carrier has had and overcome the disease, but mutual relations have been established with the organism which continues to live in the body cavity. Diphtheria bacilli usually linger in the throat after convalescence is established, and until they have disappeared the individual is more dangerous than one actually sick with the disease. Health officers have recognized this in continuing the quarantine against the disease until the organism disappears. In typhoid fever bacilli may remain in the body for a long time and be continually discharged, as in the well-known case of "typhoid Mary."[10]
Single cases of certain infectious diseases may appear in a community year after year, and at intervals the cases become so numerous that the disease is said to be epidemic. Such a disease is smallpox. This is a highly infectious disease, towards which all mankind is susceptible. Complete protection against the disease can be conferred by Jenner's discovery of vaccination. The disease becomes modified when transferred to cattle, producing what is known as cowpox, in which vesicles similar to those of smallpox appear on the skin. The inoculation of man with the contents of such a vesicle produces a mild form of disease known as vaccinia, which protects the individual from smallpox. This protection is fully as adequate as that produced by an attack of smallpox, and we are warranted in saying that if thorough vaccination, or the inoculation with vaccinia, were carried out smallpox would disappear. There are great difficulties in the way of carrying out effective vaccination of the whole population, which are accentuated by the active opposition of people who are ignorant and wilfully remain so. There exists in every state a number of people unprotected by vaccination, and among these single cases of smallpox appear. The unprotected individuals gradually increase in number, forming an inflammable material awaiting the spark or infection which produces a conflagration in the one case and an epidemic in the other.
Cerebro-spinal meningitis is another example of a disease which exists in sporadic and epidemic form. This disease is caused by a small micrococcus, the organisms joined in pairs. The seat of the disease is in the meninges or membranes around the brain and spinal cord. The micrococci enter the body from the throat and nose, and either pass directly from here into the meninges, or they enter into the blood and are carried by this into the meninges. The organisms are easily destroyed and cannot long survive the conditions outside the body, so that for infection to take place the transmission must be very direct. Carriers who have the organisms in the throat, but who do not have the disease, are the principal agents in dissemination. The mortality is high, and even in recovery permanent damage is often done to the brain or to the organs of special sense. Sporadic cases constantly occur in small numbers, and it is difficult or impossible to trace any connection between these cases. At varying intervals, often twenty years intervening, an epidemic appears which sometimes remains local in a city or state, sometimes extends to adjoining cities or states, and may even extend over a very large area. In the epidemics the mortality is much higher than in the sporadic cases. The same explanation given for smallpox cannot apply here, for there is not a similar accumulation of susceptible material. We know there is a great deal of variation in the virulence of the different pathogenic organisms, and the virulence can be artificially increased and diminished. In epidemics of meningitis the virulence of the organisms is increased, as is shown by the greater mortality. It is highly probable that such epidemics are due to changes which arise in the organisms from causes we do not know and which increase their capacity for harm. It is possible that such a change would convert a carrier into a case of disease, the organism acquiring greater powers of invasion. Such a strain of organisms arising in one place and producing an epidemic could be transported to another locality and exert the same action, or similar changes in the organisms could arise simultaneously in a number of places. Analogies to such conditions are given in plants. In certain plants it has been shown that from unknown causes there appears a tendency to the production of variations. A very beautiful herbaceous peony known as "Bridesmaid" after having grown for a number of years in single form, in one year wherever grown suddenly became double. The peculiar thing with the lower unicellular organisms is that the changes which so arise do not tend to become permanent, the organism reverts to its usual character, the disease to its sporadic type.
A very fatal form of poliomyelitis has for a number of years prevailed in Sweden. In the United States there have been continually a number of single cases of the disease, and it is not impossible that a more pathogenic strain of the organism has developed in Sweden and has been imported into this country, giving rise to the much greater extension of the disease in a number of places.